Time Delay Integration Imaging of the Nighttime Ionosphere from the ICON Observatory

One of the objectives of the Far UltraViolet (FUV) imager on the Ionospheric Connection Explorer (ICON) spacecraft is to make high resolution images of the nighttime near equatorial oxygen 135.6 nm airglow emission. This emission is largely the product of O + ion re-combination and therefore the emission intensity is a proxy for remote measurement of ionospheric density. The ICON FUV instrument is capable of high resolution imaging of the night glow by viewing the Earth’s limb from above on the left side of the spacecraft and taking rapid exposures and co-adding the resultant images for 12 seconds. To improve the resolution and compress the resulting data a new type of Time Delay Integration (TDI) technique was developed, which involves transforming the images into a distorted frame so that the displacement due to orbital motion becomes a singular constant vector for all pixels. Operating in this transformed frame it is possible to co-add and shift the images to retain the resolution and minimize the required data bandwidth. The transformation needs modeling of the object distance for all pixels. Two models, the “limb” and “sub-limb” models, are used for transforming the upper and lower parts of the ICON FUV images, respectively. At the input of the instrument there is a rotatable mirror, which allows directing the optic axis near to the plane of the local magnetic field. The images are co-added for 12 sec and are down linked and re-assembled on the ground into maps of the O emission showing an entire night pass. This is the first report on the performance of this newly developed TDI system. ICON with its low inclination (27 degree) orbit provides an extensive longitudinal coverage on each orbit complementing the coverage of GOLD or TIMED. During 179 orbits in October 2021 ICON FUV saw significant nighttime ion densities on 76% of the orbits. At low latitudes the ionization was clearly associated with the equatorial ionospheric anomaly (EIA). The maps showed significant structuring during 34% of the orbits when ICON was in the position to view the EIA. In coordinated observations GOLD and ICON FUV observed regular structuring in the form of Equatorial Plasma Bubbles (EPB-s). Comparing to GOLD observations in 2018, ICON saw significantly fewer EPB-s in the month of October 2021. ICON TDI integrated sub-limb view was tested for resolution using star images and should have seen structures less than 10 km. From the 179 orbits taken in October 2021 the shortest repetition EPB-were 350 km from peak to peaks.